The present invention relates to a color correction circuit for color representation and a color display comprising such a color correction circuit. More particularly, the present invention relates to a color correction circuit appropriate to use in such digital display devices as a plasma display panel (PDP) and a digital micro-mirror device (DMD) in which primary color light sources, different from fluorescent materials of a CRT, are used and the relation between the applied signal strength and the intensity of display is linear because the intensity of display is digitally controlled.
A color television receiver widely used at the present time uses fluorescent materials of three primary colors, which are specified by the EBU (European Broadcasting Union), and the chromaticity values of x and y of the fluorescent materials of three primary colors (red, green, blue) are different from those of the fluorescent materials (NTSC-compliant fluorescent materials) specified by the NTSC system. Since the color reproduction area of a color television receiver is narrower than that of the NTSC-compliant fluorescent materials, it is known that a distortion in color reproduction characteristics of a color television receiver is caused to occur when color video signals of the NTSC system are displayed on a color television receiver. This phenomenon is described using the UCS chromaticity diagram in FIG. 1.
FIG. 1 illustrates a distortion in color reproduction caused by the difference in the chromaticity values of x and y between the three primary color fluorescent materials (red, green, blue) of a currently used color television receiver and the NTSC-compliant fluorescent materials. In the figure, reference number 1 refers to the color reproduction area of the NTSC-compliant fluorescent materials and reference number 2 refers to the color reproduction area of a currently used color television receiver. Each circle g, yg, s, r, c, p, and b in the figure indicates green, yellow-green, ocher, red, cyan, pink, and blue, respectively, in the color reproduction area 1 of the NTSC-compliant fluorescent materials, and each bullet dot pointed by the arrow indicates a color when the NTSC signal corresponding to the color is displayed on a currently used color television receiver, in other words, the reproduced color in the color reproduction area 2. The arrow indicates the shift in position in the UCS chromaticity diagram between a reproduced color in the color reproduction area 1 and that in the color reproduction area 2 due to the distortion in color reproduction. The double circles yg and b indicate that the reproduced colors are not influenced by the distortion.
As shown in the figure, there exists a difference between the color reproduction area 1 of the NTSC-compliant fluorescent materials and the color reproduction area 2 of a currently used color television receiver. The distortion of the color reproduction characteristics in a color television receiver caused by this difference occurs in the way that the distortion causes most reproduced colors to move toward the axis line 16 that connects yellow-green yg and blue b, and for example, green g or ocher s is compressed to yellow-green yg by the distortion in color. As described above, the distortion in color reproduction does not occur irregularly, instead occurs in the way that the distortion causes the position of a color reproduced by the NTSC-compliant fluorescent materials to move from both sides of an axis line (axis line 16 in FIG. 1) of some hues toward the axis line. In this way, the color reproduction characteristics of a color television receiver are degraded.
An example of the conventional art that solves this problem is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-23478. This disclosed conventional art is briefly described below.
As mentioned above, a distortion in color reproduction occurs in the direction toward an axis line in a currently used color television receiver. The conventional art corrects the distortion and improves the color reproduction characteristics by enlarging the amount of the change in the color so that the position of the color deviates from the axis line. In other words, as shown in FIG. 1, the conventional art corrects the compression in the direction toward the axis line 16 and cancels the distortion of color reproduction by enlarging the amount of the change in the color so that the position of the color deviates from the axis line 16, that is, in the directions shown by the arrows 17 and 18.
This correction method is described using FIG. 2. In the figure, the horizontal axis is marked with phases of input chromatic signals of a color television receiver and the vertical axis is marked with phases of the corrected chromatic signals. When not corrected, the relation in phase of these chromatic signals is as shown by a dotted line, on the other hand, when corrected by the above-mentioned conventional art, the relation in phases of these chromatic signals is as shown by a solid line. By this correction, the color change is caused in the directions shown by the arrows 17 and 18 in FIG. 1 as mentioned above.
FIG. 3 shows a block diagram that illustrates an example of a conventional color reproduction correction device that corrects the distortion in color reproduction by adjusting the hue in the direct current control method, as mentioned above. In the figure, reference number 3 refers to a band amplifier, number 4, a reference color carrier oscillator, number 5, a phase shifter, number 6, a 90° phase advancer (+90°), number 7, a hue adjuster, number 8, a limiter, numbers 9 and 10, phase detectors (P.D.), number 11, a clipper, number 12 a multiplier, number 13 a direct current power source for hue adjustment, number 14, an adder, and number 15, a color demodulator circuit.
In FIG. 3, the chromatic signals of the received color video signals are limited in bandwidth by the band amplifier 3 and are supplied to the phase detectors 9 and 10 via the limiter 8, as well as to the color demodulator circuit 15. The burst signals taken from the band amplifier 3 are supplied to the reference color carrier oscillator 4 and a reference color carrier synchronized with these burst signals in phase is obtained. After being shifted in phase in the phase shifter 5, the reference color carrier is supplied directly to the phase detector 9, and at the same time supplied to the phase detector 10 after the phase is advanced in the 90° phase advancer 6. If the characteristics of the phase shifter 5 are appropriately selected so that the phase of the output reference color carrier is yellow green, that is, 5° with respect to the phase of the burst signal of the input chromatic signal, the phase detector 9 is a phase detector for yellow green, and the phase detector 10, a phase detector for the axis perpendicular to the yellow green signal. If we assume that the phase of yellow green signal is the reference phase, and the phase of the chromatic signal with respect to the reference phase is θ, the voltage level of the output signal V1 of the phase detector 9 will change with respect to phase θ as shown by the curve V1 in FIG. 4A, and that of the output signal V2 of the phase detector 10, as shown by the curve V2 in FIG. 4B. The output signal V1 of the phase detector 9 is clipped by the clipper 11 at a specified clip level, and signal V3, which has the voltage characteristic with respect to phase θ as shown by the curve V3 in FIG. 4C, is obtained. Thus the output signal of the clipper 11 is adjusted appropriately so that the corrections 17 and 18 in the vicinity of yellow green as shown in FIG. 1 are obtained. Here, for example, the clipping level of the clipper 11 is selected so that the range is between −60 and +60°. The output signal V3 of the clipper 11 is multiplied by the output signal V2 of the phase detector 10 in the multiplier 12. Thus the voltage level of the output signal V4 of the multiplier 12 forms the curve V4 as shown in FIG. 4D. The output signal (voltage) V4 of the multiplier 12 and the direct voltage Vd of the direct current power source 13 are added in the adder 14 and supplied as a control voltage to the hue adjuster 7.
The hue adjuster 7 changes the phase of the reference color carrier from the reference color carrier oscillator 4 according to the control voltage from the adder 14. The change in phase is, for example, conducted by adding the color sub-carrier and the shifted one of the color sub-carrier, which is shifted by 90°, in a proportion according to the control signal. The change in phase of the color sub-carrier caused by the hue adjuster 7 is controlled using the control voltage from the adder 14, that is, the output voltage Vd of the direct current power source 13 and the output voltage V4 of the multiplier 12. If it is assumed that the vertical axis is marked with values of the output voltage Vd of the direct current power source 13 and the horizontal axis is marked with values of the phase Δθ (delta theta) of the output color sub-carrier of the hue adjuster 7 with respect to the reference color carrier of the reference color carrier oscillator 4, the phase Δθwill appear, as shown in FIG. 4E, with respect to the output voltage Vd of the direct current power source 13. Since the control voltage of the hue adjuster 7 is the sum of the output direct current voltage Vd of the direct current power source 13 and the output voltage V4 of the multiplier 12, which has the characteristic as shown in FIG. 4D, if the output voltage V4 of the multiplier 12 changes in the range between −e and +e in FIG. 4D, the output voltage of the adder 14 changes in the range between the specified direct voltage Vd from the direct current power source 13 +/−e, therefore, the phase Δθ is Δθ0 for the direct current voltage Vd when the phase of the chromatic signal is outside the range between −60° and +60°, and that changes in the range between the Δθ1 and the Δθ2 (Δθ1<Δθ0<Δθ2), when the chromatic signal is within the range between −60° and +60°. Then, when the reference color carrier from the hue adjuster 7 is supplied to the color demodulator circuit 15, and the chromatic signal from the band amplifier 3 is demodulated, the signal of the color, of which the amount of the change in the color in the vicinity of yellow-green is enlarged, is demodulated as shown in FIG. 2 and the correction of the distortion in color reproduction is carried out as shown in FIG. 1.
By the way, in the above-mentioned conventional art there exist the following problems:    1. A number of demodulation axes are necessary to improve and attain the color reproduction with less distortion because the color reproduction is corrected by using plural demodulation axes best fit to the specific hues and combining the selected hue ranges that found to be appropriate. Therefore, a problem may occur that the level of the color reproduction improvement is in proportion to the scale of the correction circuit, and it is almost impossible to match the color in a correction circuit of a small scale.    2. When plural signal systems, to which different chromaticity values of x and y are assigned, are to be dealt with, plural correction circuits are required, accordingly, resulting in an increase in the circuit scale.    3. Because the targets to be corrected are color demodulator circuits and color difference signals, it is difficult to deal with the input signals of the three primary colors (RGB signals).    4. Because the demodulation gain of R-Y is increased in order to improve the hues of red, magenta, and cyan, the input of red is saturated and the collapse of red occurs.